1. Field of the Invention
This invention relates to a zoom lens suitable for use in an image pickup apparatus (camera) such as a television camera, a video camera, a photographic camera or a digital camera, and particularly to a zoom lens of a great aperture ratio and a high magnification ratio using the so-called internal focus system (inner focus system) for effecting focusing by some lens units in a first lens unit being moved, thereby having high optical performance over the whole objective distance.
2. Related Background Art
In recent years, in image pickup apparatuses such as television cameras or video cameras, the downsizing and lighter weight of the entire apparatus have been contrived by using a compact solid state image pickup element in order to enhance the operability and mobility of the apparatus. Correspondingly thereto, similar downsizing, lighter weight, and higher specification have also been contrived in zoom lenses used in these image pickup apparatuses.
Among the zoom lenses, a four-unit zoom lens comprising, in succession from the object side, four lens units, i.e., a first lens unit for focusing having a positive refractive power, a second lens unit for focal length change having a negative refractive power, a third lens unit having a positive or negative refractive power for correcting an image plane fluctuating with focal length change, and a fourth lens unit for imaging having positive refractive power is rotatively easily permitted to have a higher magnification and a greater aperture and therefore is often used as the zoom lens of a color television camera for broadcasting.
In the zoom lenses in the latest television cameras and video cameras, the shorter focus of the wide angle end, the longer focus of the telephoto end and a higher variable power ratio have been strongly required. Also, it has become one of the important factors in specification and photographing effect to shorten the shortest photographing distance (MOD).
In zoom lenses, a lens construction in which the power (refractive power) of each lens unit has been strengthened has heretofore been often used in order to achieve downsizing, lighter weight and higher specification. If in this case, an attempt is made to strengthen the power of a first lens unit of positive refractive power for focusing to thereby achieve downsizing and lighter weight, the fluctuations of the various aberrations resulting from a change in the object distance will pose a problem. Particularly, the fluctuations of spherical aberration, astigmatism and chromatic aberration will become great and the optical performance will be remarkably reduced. These fluctuations of the aberrations by focusing become more remarkable as the focal length is longer, that is, in a lens system wherein F number is smaller and which is brighter. Therefore, various propositions have been made in the focusing systems.
Among the focusing systems, various apparatuses adopting the internal focusing system in which some other lens units in the lens system than a first lens unit on the object side are moved to thereby effect focusing have heretofore been proposed.
Generally in a zoom lens of the internal focusing type, as compared with a zoom lens in which a first lens unit is moved to effect focusing, the effective diameter of a first lens unit can be made small and the shortening of the shortest photographing distance is easy and further, focusing is effected with a relatively small lens unit moved, and this leads to the features that the drive force for lens units may be small and that quick focusing can be done.
As such a zoom lens of the internal focusing type, for example, Japanese Patent Publication No. 52-41068 proposes a four-unit zoom lens comprising, in succession from the object side, four lens units, i.e., a first lens unit of positive refractive power, a second lens unit of negative refractive power for focal length change, a third lens unit of a negative refractive power for correcting the movement of an image point resulting from focal length change, and a fourth lens unit of a positive refractive power, wherein some lens units in the first lens unit which are adjacent to the image plane are moved to effect focusing.
To obtain a great aperture ratio and a high variable power ratio and moreover high optical performance over the entire variable power range and the entire focus range in a zoom lens, it is necessary to appropriately set the refractive power of each lens unit, the construction of the lens, the aberration apportionment, the achromatism apportionment, etc.
For example, to obtain high optical performance in which the fluctuations of aberrations are small over the entire variable power range and the entire focus range, it becomes necessary in many cases to increase the number of the constituent lenses in each lens unit and increase the degree of freedom in aberration correction. Therefore, if an attempt is made to achieve a zoom lens of a great aperture ratio and a high variable power ratio, there will unavoidably arise the problem that the number of lenses increases and the entire lens system becomes bulky.
Also, if MOD is shortened, there will arise the problem that the fluctuations of aberrations, particularly the fluctuation of spherical aberration at the telephoto end, during focusing, becomes great and it becomes very difficult to contrive the downsizing of the entire lens system over the whole object distance from an infinity object to a short distance object, and yet obtain high optical performance.
So, as one of means for solving these problems, there have been proposed some examples adopting the aforedescribed internal focusing system.
However, the focusing system proposed, for example, in the aforementioned Japanese Patent Publication No. 52-41068 has not always been of a sufficient lens construction for a zoom lens for broadcasting of which a great aperture ratio, a high variable power ratio, shorter MOD and a high specification are required. Particularly, it has not always been sufficient as a zoom lens for broadcasting in which the apportionment of refractive power to a lens unit fixed during the focusing of a first lens unit and a lens unit axially moved for focusing and the refractive power, the dispersion value, etc. of the material of each lens are of a high specification.
Generally, to achieve the downsizing and lighter weight of an entire zoom lens which are the recent user's demands and yet contrive a higher specification, it is necessary to appropriately set the refractive power and lens construction of each lens unit. Particularly in a four-unit zoom lens, it is an important factor how well balancedly the lens construction of a first lens unit (front lens) which most governs the size and weight of the entire lens system should be designed relative to the lens construction of the entire zoom lens system.